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Chapter 5

Chemistry10 Chapter Five.doc

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Queen's University
CHEM 222
Richard Ascough

Chemistry Chapter Five The Nature of Aqueous Solutions Reactions in aqueous (water)solution are important because: (1) water is inexpensive and is able to dissolve a vast number of substances; (2) in such solutions, many substances are dissociated into ions which can participate chemical reactions; and (3) these solutions are found everywhere, from seawater to living systems. Unlike metallic conductors in which electrons carry the electric charge, the electricity conducted in aqueous solutions is carried by the ions. When a solute dissociates into ions in an aqueous solution and becomes and electric conductor, it is known as an electrolyte. ***Pure water contains so few ions that it does not conduct an electric current. *** Based on how well a solution conducts electricity, we can deduce the strength of the presence of ions. We can label a solute as a non- electrolyte, strong electrolyte, or weak electrolyte. A non-electrolyte is a substance that is not ionized and does not conduct electric current (e.g. the lamp fails to light up). Therefore, there are no ions or extremely low concentration of ions. A strong electrolyte is a substance that is essentially completely ionized in aqueous solution, and the solution is a good electrical conductor (e.g. the lamp lights up brightly) and thus, has a high concentration of ions. A weak electrolyte is partially ionized in aqueous solution and the solution is only a fair conductor of electricity, thus, the concentration o f ions in the solution is low (e.g. the lamp lights up only dimly). When determining if a solution is more likely to be a strong electrolyte, weak electrolyte or non-electrolyte, it is best to remember this generalization: • Essentially all soluble ionic compounds and only a relatively few molecular compounds are strong electrolytes. • Most molecular compounds are either non-electrolytes or weak electrolytes. Some examples of a strong electrolyte are: HCl, NaOH and KBr. Some examples of a weak electrolyte are: HF, CH CO3H. Some examples of non-electrolytes are: H O 2 and CH O3. If a solution contains strong electrolytes, the equation is written with the arrow of the reaction going in one direction, usually right. This indicates that the ionization in water is complete. 2+ - MgCl (2) (H 0) 2g (aq)+ 2Cl (aq) In a situation where the solution is characterized as a weak electrolyte is best described as a reaction that does not go to completion. In these cases, only a portion of the solute molecules in the solution are ionized. The double arrows indicate that the process is reversible. This means that while the forward reaction is taking place, the reverse action is also occurring and its products are the reactants of the forward reaction. HC H2O 3H 2 + (aq)+ C H2O 3 2 (aq) As for a non-electrolyte solution, we would imply write the molecular formula (e.g. CH O3 (aq) Relative Concentration in Solutions In fully dissociated reactions such as the decomposition of magnesium chloride as state above, the concentration is derived from the number of ions present in the equation for each element based on one elements statistics. For example, as in the decomposition of magnesium chloride, assume that there is 0.0050 M of MgCl to 2 decompose and we were to find the concentration of its products. We would write it as the following: [Mg 2+] = 0.0050 M and [Cl ] = 0.0100 M. The square brackets indicate concentration. The reason [Cl ] is twice the M of [Mg ] is because there are - 2+ 2 ions of Cl for every Mg ion. Problem: What are the aluminum and sulfate ion concentrations in 0.0165 M Al (SO ) ? 2 4 3(aq) Solution: Identify the solute as a strong electrolyte and write an equation to represent its dissociation. Al2(SO 4 3(s)(H O2 2 Al 3+(aq)+ 3SO 42(aq) 3+ [Al3+] = [(0.0165 mol Al (SO ) 2 1 L4 3 (2 mol Al / 1 mol Al (SO ) )] = 0203304 3l 3+ Al / 1 L = 0.0330 M [SO 4 = [(0.0165 mol Al (SO ) 2 1 L)4 3(3 mol SO / 1 mol Al 4SO ) )] = 0.0295 mo4 3 SO /41 L = 0.0495 M Precipitation Reactions Precipitation reactions occur when certain cations and anions combine to produce an insoluble solid called precipitate. Solubility Rules: + + • Compounds that ARE soluble: Alkali metal ions and ammonium salt (Li , Na , + + + + - K , Rb , Cs and NH ); Nit4ates (
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